move talloc context creation out of on_dso / constructors

the various constructors get called in a non-obvious, linker determined
order, which makes certain objects disappear from the talloc report.

This change moves the talloc context creation into a new talloc_ctx.c file
diff --git a/openbsc/src/vty/vty.c b/openbsc/src/vty/vty.c
index 53800d1..affe28d 100644
--- a/openbsc/src/vty/vty.c
+++ b/openbsc/src/vty/vty.c
@@ -1630,7 +1630,7 @@
 /* Install vty's own commands like `who' command. */
 void vty_init()
 {
-	tall_vty_ctx = talloc_named_const(tall_bsc_ctx, 1, "vty");
+	tall_vty_ctx = talloc_named_const(NULL, 0, "vty");
 
 	/* For further configuration read, preserve current directory. */
 	vty_save_cwd();